Learning outcomes
- Explain the key principles of contactors, relays and motor starters.
- Carry out a structured practical check or configuration task.
- Recognise common faults, risks and evidence needed for diagnosis.
Module 2 · Lesson 2
Contactors, relays and motor starters
Understand contactor logic, auxiliary contacts, electrical interlocks, overloads and starter arrangements.
Before you begin
De-energised starter panel or training simulator.
Safety: Use site procedures, approved test equipment and competent supervision. Never bypass a protection or interlock without formal control.
Step-by-step learning
- Identify main poles, coil voltage, auxiliary contacts and overload reset.
- Trace start, stop, seal-in and trip circuits.
- Check forward/reverse or star/delta interlocks for both electrical and mechanical protection.
- Simulate each permissive and verify the starter cannot energise under unsafe conditions.
Ian's practical tips:
- A seal-in contact explains many “starts then immediately stops” faults.
- Measure coil voltage at the coil terminals, not only at the control transformer.
Fault-finding questions
- What should be happening, and what evidence proves it?
- Where is the last known correct signal, voltage, state or process condition?
- What changed immediately before the fault?
- Can the system be divided into smaller testable sections?
Practical activity
Create a one-page test record for this lesson. Include expected values, measured results, equipment used, risks controlled, defects found and the final proven condition.